The applicability of mass spectrometric methods to the qualitative and quantitative analysis of peptides in biological material has been evaluated. Two general methodologies have been developed: one involves cleaving of the peptides with the enzyme Depeptidyl-amino peptidase I (D. A. P. I) and gas chromatographic-mass fragmentographic detection of the dipeptides thus generated; the other consists of selected ion monitoring of the fragments arising from a derivative of the whole peptide molecule, using the solid inlet of the instrument as a sample entry port.